1. Field of the Invention
The present invention relates to an electronic circuit unit built in small-sized electronic apparatuses such as mobile phones and PDA (Personal digital Assistants), and to its shield structure.
2. Description of the Related Art
In general, small-sized electronic apparatuses such as mobile phones and PDA have a built-in electronic circuit unit, which is mounted with many circuit devices on a printed circuit board. The circuit devices include the following circuit device groups. One is a circuit device group such as CPU (Central Processing Unit) and DSP (digital signal Processor) radiating a noise. Another is a circuit device group such as radio circuit, which is easy to receive an influence by noise. In the foregoing circuit device groups, the noise radiated from the circuit device group such as CPU gives an influence to the circuit device group such as radio circuit depending on the positional relationship. As a result, this is very unfavorable because radio transmitting and receiving performance is reduced.
Conventionally, the following structure has been employed. For example, two frames produced by sheet metal processing are prepared. The circuit device group radiating the noise is enclosed with one of two frames; on the other hand, the circuit device group readily receiving the influence by noise is enclosed with the other thereof. The structure described above is given, and thereby, each frame functions as a shield case; therefore, the influence by the noise is reduced between the foregoing circuit device groups.
However, if the foregoing two frames are located separately from each other, a clearance area for repair must be secured between these frames. For this reason, these frames must be arranged with a given distance or more. As a result, the electronic circuit unit is inevitably configured into a large size. This is difficult to meet the requirements such as miniaturization of electronic apparatus and high packaging density of circuit device resulting from high function of apparatus.
Moreover, the shield structure given below has been proposed. According to the shield structure, a partition plate is provided in a rectangular frame to form two independent rooms. The foregoing two rooms each receive the circuit device group radiating the noise and the circuit device group readily receiving the influence by noise. The technique is detailedly disclosed in JPN. PAT. APPLN. KOAKI Publication No. 2001-144487.
However, according to the foregoing shield structure, the frame is configured to a large size. If the frame is produced using sheet metal processing, the flatness of the frame is reduced. This is a factor of causing packaging failure when the frame is mounted on a printed circuit board. Moreover, if the upper opening portion of the frame is attached with a cover, the frame and the cover are formed into a large size; for this reason, a gap is readily formed between the frame and the cover. This is a factor of reducing the uniformity of isolation characteristic between circuit devices.